In a typical polymerisation reaction, monomer, diluent, catalyst, co-catalyst and optionally co-monomer and hydrogen are fed to a reactor where the monomer is polymerised. The diluent does not react but is typically utilised to control solids concentration and also to provide a convenient mechanism for introducing the catalyst into the reactor.
Following such polymerization process, a polymerisation effluent is produced comprising slurry of polymer solids in a liquid that contains diluent, dissolved unreacted monomer, and dissolved unreacted co-monomer. Typically, this liquid also includes traces of heavier elements, e.g. oligomers, and lighter components including H2, N2, O2, CO and/or CO2. Catalyst will generally be contained in the polymer.
The polymer is separated from the liquid by techniques such as flash vaporisation. Afterwards, it is highly desirable to further treat the vapors in order to recover the unreacted monomer, unreacted co-monomer and the diluent, since there is an economic interest in re-using these separated components including the monomer, co-monomer, and the diluent, in a polymerization process.
It is well known in the art that separation of a vaporous stream comprising unreacted monomer, unreacted co-monomer and diluent issued from the effluent of a polymerization process may be treated in a distillation system for separation of its components. U.S. Pat. No. 4,589,957 for instance describes a separation process of a hydrocarbon-containing vaporous stream comprising monomer, co-monomer and diluent issued from the effluent of a homo-polymerization and/or co-polymerization process. The described process comprises subjecting the vaporous stream to two-stage distillation provided with a common accumulation zone wherein the condensate from the accumulation zone serves as the source of feed for the second distillation and reflux for the first distillation.
In view of re-using the separated components issued from the effluent of a polymerization process in said process, it is important, that the separation process of a vaporous effluent stream is done in a manner such that high purity streams of monomer, co-monomer and diluent are separately recovered. It is also highly required to substantially eliminate the lighter components, as those indicated above, from such vaporous effluent stream, since recycling of such lighter components to the polymerization process could seriously reduce polymerization efficiency and induce sub-optimal polymerization conditions.
However, a major problem encountered in many distillation systems, is that distillation columns used in such systems show stability problems. Distillation conditions in such columns may undergo important oscillations in temperature and pressure values, resulting in a fluctuation of their operation conditions. As a result thereof, separation of the different components of vaporous effluent streams is sub-optimal.
Another problem related to distillation systems is a sub-optimal separation of lighter components, including H2, N2, O2, CO and/or CO2, from diluent. As a consequence, use of diluent containing these components in a polymerization process may result in sub-optimal polymerization conditions.
Yet another problem associated with distillation systems for separating vaporous polymerization effluent streams is the high amount of energy which is required for carrying out the separation process.
In view of the above, it is clear that there remains a need in the art for providing more accurate systems for separating vaporous hydrocarbon containing effluent streams, for instance those issued from a polymerization process.
It is therefore an object of the present invention to provide a process for optimizing the separation of a hydrocarbon-containing feed stream.
It is further an object of the present invention to provide a process for improving the operation of a distillation system. More in particular, it is an aim to provide a process for improving stability of a distillation system.
It is yet another object of the present invention to provide a process for maximizing the separation of lighter components from other components comprised in a hydrocarbon feed stream.
A further object of this invention is to provide a process for reducing the energy consumption of a distillation system.
Yet another object of the present invention is to provide a process wherein more olefin-free hydrocarbon diluent is recycled than the amount that can be re-used in a polymerization reaction.